The invention relates to an exhaust gas system for industrial gas turbines with an exhaust gas conduit and a chimney connected to it, as described in the preamble to claim 1. Residential zones and installations which are operated by gas turbines, such as combined heat and power stations, are becoming increasingly close together. In order to keep the noise annoyance to the population at a low level, noise emission restrictions have become more and more severe in recent years. In many places, restrictions on low-frequency noise have been introduced in addition to the existing restrictions on high and medium frequencies. The noise emission from a gas turbine installation principally takes place via its exhaust gas system. The occurrence of the low-frequency noise, which is difficult to deal with, has many causes and may be attributed inter alia to pulsations in the combustion space.
So that restrictions on low-frequency noise emissions can be met, absorption noise suppressors have been installed in the exhaust gas system of gas turbine installations, as is mentioned for example in DE-A1-44 19 604 and DE-A1-40 09 072. This is intended to reduce the low-frequency noise at the location at which its radiation into the surroundings takes place. Whereas, however, noise in the high and medium frequency ranges can be relatively successfully absorbed with absorption noise suppressors, low-frequency noise is difficult to deal with because conventional noise suppressors only exhibit a slight noise suppression effect at low frequencies. In order to permit reduction in low-frequency noise, it is therefore necessary to install large absorption noise suppressors with suppression mats of up to 800 mm thickness in the exhaust gas system of the installation. This increases the space requirements of the exhaust gas installation, reduces its power in some circumstances because of the pressure drop in the system and is, in addition, very complicated with respect to assembly and maintenance. In consequence, the exhaust gas system becomes very expensive.
The object of the invention is therefore to create an exhaust gas system of the type mentioned at the beginning in which low-frequency noise emissions are efficiently reduced without the power of the installation being essentially impaired and which, in addition, is simple and economical with respect to assembly and maintenance. This object is achieved by means of an exhaust gas system with the features of claim 1. In an exhaust gas system for industrial gas turbines, an exhaust gas conduit and a chimney connected to it together form a continuous flow duct. A Helmholtz resonator is acoustically coupled on the flow duct in the exhaust gas system. The Helmholtz resonator is precisely tuned to the low frequency which has to be suppressed. For this purpose, it demands less space than an absorption noise suppressor. The assembly of a Helmholtz resonator is very simple and, at large flow velocity, its useful life is much higher than that of absorption noise suppressors. In addition, the employment of Helmholtz resonators does not cause any decrease in the power of the installation. For these reasons, the exhaust gas system can be more easily assembled and maintained and the overall installation can be operated more economically.
If the inlet opening of the Helmholtz resonator is located in the region of the pressure maximum of an acoustic mode in the exhaust gas system, its efficiency is at a maximum.
It is very advantageous to locate the Helmholtz resonator in the transition region between the exhaust gas duct and the chimney because, as a rule, there are hardly any space problems in this area. It is particularly favorable to provide the Helmholtz resonator on the chimney rear wall, which bounds the exhaust gas duct in the flow direction, because this permits particularly simple assembly.
In a preferred embodiment, the dimensions of the exhaust gas duct and the chimney are selected in such a way that a pressure maximum of the acoustic mode occurs in the transition region between the exhaust gas duct and the chimney. In this way, the Heimholtz resonator can be very simply assembled, as described above, and is in addition extremely efficient.
Thermal insulation of the Helmholtz resonator from the outside ensures an approximately constant temperature of the Helmholtz resonator and, therefore, frequency stability of its absorption properties.
If the Helmholtz resonator has a throat which can be adjusted in its length and/or its cross section, the Helmholtz resonator can be better adjusted to the frequencies to be absorbed.
In a further preferred embodiment, the Helmholtz resonator has an adjustable volume. This again provides a simple possibility for matching to the frequencies to be absorbed. The adjustable volume can be very simply realized if the height of the side walls is arranged to be adjustable by means of a displaceable base.
The Helmholtz resonator can be matched particularly simply to the frequency to be absorbed if its temperature is adjustable. The temperature adjustment capability can, for example, be simply realized by attaching heating elements to the outer walls of the Helmholtz resonator. Another low-cost possibility consists in designing the Helmholtz resonator so that medium can flow around it in such a way that, for the purpose of temperature regulation, either hot exhaust gas is branched from the exhaust gas system and guided around the outer walls of the Helmholtz resonator or cold air flows around the latter.
In a further preferred embodiment, the Helmholtz resonator is screened in an acoustically transparent manner from the flow in the flow duct. This permits improved noise absorption by the Helmholtz resonator. Such screening can be very simply and expediently realized by means of an absorption noise suppressor located between the inlet opening of the Helmholtz resonator and the flow.
It is particularly advantageous to use an absorption noise suppressor which has the following approximate construction: A first perforated cover is part of a wall bounding the flow duct. A flow-resistant fabric and a layer of absorption material, which is located on the side of the perforated cover facing away from the flow duct, adjoins this first perforated cover. A second perforated cover follows this layer of absorption material on the side facing away from the flow duct. The absorption noise suppressor is laterally enclosed by side walls. Such an absorption noise suppressor can accept loads satisfactorily when bounding a flow duct with high flow velocities.
If a hollow space is arranged between the absorption noise suppressor and the inlet opening of the Helmholtz resonator, this has a positive effect on the vibration behavior of the Helmholtz resonator and therefore on its absorption capability.
It is very advantageous to provide a plurality of Helmholtz resonators in the exhaust gas system. These can then be located at different locations in the exhaust gas system, for example where respective maxima of the sonic modes occur. They can also be tuned to different low frequencies and, in this way, contribute to an even more effective reduction in the low-frequency noise. For this purpose, they can be located at different locations in the exhaust gas system or also close together. In order to ensure good noise absorption, however, the Helmholtz resonators should be separated from one another in a gas-tight manner.
Other preferred embodiments are the subject matter of further sub-claims.